This invention relates generally to apparatus and methods for applying a force to a length of fiber optic material and particularly to apparatus and methods for compressing an optical fiber to control the refractive indices of the fiber and, therefore, the polarization of the optical wave propagating in the fiber. Still more particulary, this invention relates to a structural frame for a fiber squeezer and method of assembly thereof for applying compressional forces transverse to the length of an optical fiber to produce stress-induced birefringence for controlling the polarization of light guided by the fiber.
Some familiarity with polarization of light and propagation of light within an optical fiber will facilitate an understanding of the present invention. It is well-known that a light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The normal modes are directly related to the distributions of energy within the structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate essentially unattenuated are the normal modes. Waves that are severely attenuated are generally called "evanescent modes". A single mode fiber will propagate only one spatial distribution of energy for a signal of a given frequency.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. If only the electric field vector is perpendicular to the direction of propagation, which is usually called the optic axis, then the wave is said to be a transverse electric (TE) mode. If only the magnetic field vector is perpendicular to to the optic axis, the wave is a transverse magnetic (TM) mode. If both the electric and magnetic field vectors are perpendicular to the optic axis, then the wave is a transverse electromagnetic (TEM) mode. None of the normal modes require a definite direction of the field components; and in a TE mode, for example, the electric field may be in any direction that is perpendicular to the optic axis.
The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for each mode. If all the electric field vectors in a wave point in only one particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude, the electric field is circularly polarized because the net electric field is then a vector that rotates around the optic axis at an angular velocity equal to the frequency of the wave. If the two linear polarizations are unequal, the wave has elliptical polarization. In general, any arbitrary polarization can be represented by either the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two oppositely directed elliptical having orthogonal semi-major axes.
The velocity of an optical signal depends upon the index of refraction of the medium through which the light propagates. Certain materials have different refractive indices for different polarizations. A material that has two refractive indices is said to be birefringent. The polarization of the signal propagating along a single mode optical fiber is sometimes referred to as a mode. A standard single mode optical fiber may be regarded as a two mode fiber because it will propagate two waves of the same frequency and spatial distribution that have two orthogonal polarizations.
Two different polarization components of the same normal mode can propagate through a birefringent material unchanged except for a difference in velocity of the two polarizations. Circular birefringence, linear birefringence, and elliptical birefringence are each described with reference to different polarization modes. If a material exhibits circular birefringence, the polarization of a light wave is expressed as a combination of two counter-rotating components. One of the circular polarizations is referred to as "right-hand circular" while the other is referred to as "left-hand circular". In a non-birefringent material both right hand and left hand circular polarizations travel at the same velocity. The counterrotating electric field vectors of the circularly polarized components of the light represent the polarization modes for circular birefringence. If the light is linearly polarized, the circular polarization vectors are in phase with one another and are of equal amplitude. If the light is elliptically polarized, the circular polarization vectors are of unequal amplitudes. In general, elliptically polarized light may have varying degrees of ellipticity; and the polarization may range from linearly polarized at one extreme to circularly polarized at the other extreme.
In a circularly birefringent material, the velocity of propagation of one circular polarization vector is greater than the velocity of propagation of the counterrotating polarization vector. Circular birefringence causes a wave to rotate or twist as it propagates through the medium. Similarly, in a material that is linearly birefringent, the propagation velocity of the light in one of the linearly polarized modes is greater than the propagation velocity of the light in the other normal linearly polarized mode. Elliptical birefringence results when both linear birefringence and circular birefringence exist at a point in a material through which the light wave is propagating. The elliptical birefringence affects the polarization of light in a complex manner which depends, in part, upon the relative magnitudes of the linear birefringence and the circular birefringence.
In summary, any polarized light can be represented by two circularly polarized waves having proper phase and amplitude. Alternatively, the light could be represented by either elliptically rotating components or by perpendicular linearly polarized components of electric field.
There are a number of birefringent materials. For example, depending on their structure and orientation to the light propagating through it, certain crystals are circularly birefringent; and other crystals are linearly birefringent. Other types of crystals, for example quartz, can have both circular birefringence and linear birefringence so as to produce elliptical birefringence for a light wave propagating in a properly chosen direction.
The amount of birefringence is used herein to mean the difference between the two refractive indices of a medium that guides a light wave. Controlling the amount of birefringence permits the control of the polarization of a light signal output from a length of fiber optic material. If the wave propagated by a fiber comprises two linear polarization components, increasing or decreasing the difference between the refractive indices of the fiber provides means for controlling the optical length of the fiber for each of the two polarizations. If the fiber is birefringent, then the two polarization components will be shifted in phase as they propagate along the fiber. Since the velocity of light in an optical fiber is v=c/n, where c is the free space velocity of light and n is the refractive index of the fiber, the polarization component having the lower refractive index will have a smaller transit time in the fiber than will the component having the higher refractive index. A birefringent medium therefore rotates the plane of polarization on an optical signal propagating therein.
It is well known that application of a compressive force to a length of optical fiber along an axis transverse to the fiber changes the refractive indices of the fiber by means of the photoelastic effect, resulting in stress-induced birefringence. Various devices for applying transverse compressive forces to optical fibers are known in the art. One such device is disclosed in SPIE Vol. 425, pp. 111-113 by DePaula et al. That device includes an optical fiber disposed between two quartz piezoelectric transducers driven in phase by an electrical signal. When the driving signal is zero, the fiber is unstressed. When the driving signal is not zero, the fiber is squeezed between the quartz plates and experiences a change in birefringence. Therefore, controlling the force applied to the fiber controls the amount of birefringence.
Another method for controlling the polarization of a light wave is disclosed in U.S. Pat. No. 4,389,090, issued June 21, 1983 to LeFevre, assignee to the Board of Trustees of Leland Stanford Jr. University. LeFevre discloses several embodiments of a polarization controller, all of which employ one or more lengths of optical fiber formed in a coil of relatively small radius to stress the fiber sufficiently to induce birefringence. Rotation of the planes of the coils through predetermined angles produces a controlled change in the polarization of light guided by the fiber.
Construction of a device such as a fiber optic rotation sensor requires precise control of the polarization of the optical signals guided by an optical fiber.
The coils disclosed by LeFevre provide adequate polarization control in some cases, however, such polarization controllers are best suited for use in static situations where the polarization is to be changed only by a predetermined amount. In a fiber optic rotation sensor two counter propagating beams of the same polarization propagate through a coiled portion of optical fiber. Optical polarizers eliminate unwanted polarizations from the system. The signal input to each polarizer first passes through a polarization controller to ensure that only light of the desired polarization is input to the polarizer to avoid unnecessary loss of signal intensity. The polarization controller of LeFevre changes any input polarization by a predetermined amount so that if the polarization of the input to the polarization controller changes, then the polarization of the signal output from the polarization controller also changes.
U.S. patent application Ser. No. 557,844 by George A. Pavlath, assignee to Litton Systems, Inc. discloses a fiber optic system including a polarizer that guides light of a desired polarization in a fiber and radiates light of an undesired polarization from the fiber. The radiated signal is incident upon a photodetector, which produces an error signal that is amplified before being input to polarization controller comprising a plurality of fiber squeezers. The fiber squeezers comprise piezoelectric actuators that apply stresses to the fiber to control the polarization of light impinging upon the polarizer. The system minimizes the error signal so that the polarization of the signal input to the polarizer is essentially the desired polarization.
Difficulties have arisen in the construction of fiber squeezers suitable for forming a polarization controller for providing light of the desired polarization for input to the polarizers in an optical rotation sensing system. To provide the desired degree of polarization control with fiber squeezers requires preloading the fiber. Application of a suitable electrical signal to an actuator permits the loading on the fiber to be either increased or decreased, which provides complete control of the birefringence of the fiber. However, the allowable deformation at room temperature is in the region of 1.0 percent before fracture occurs. For a 75.mu. diameter glass fiber, this deformation is only about 0.75.mu.. Previous devices for preloading the fiber have used wedges or screws, which lack the required precision and sensitivity to preload the fiber without substantial risk of fracturing the fiber. One such device includes an adjusting screw having about eighty threads per inch aligned with a piezoelectric actuator formed of PZT. The piezoelectric actuator acts upon a fiber held between two pressure pads. Turning the screw to advance it toward the fiber stresses the fiber.
A second such device includes a frame having a screw driven wedge oriented such that a planar surface of the wedge contacts a second wedge that holds a PZT transducer adjacent a pressure pad, which transmits force from the transducer to the fiber. Turning the screw to advance the wedge into the frame perpendicularly to the fiber compresses the transducer against the fiber to provide a preload.